The present invention relates to electron-beam furnaces for melting materials by electron bombardment in a high vacuum ambient and, more particularly, to electron-beam furnaces in which the material is melted and refined in a large-area, trough-shaped hearth scanned by one or more high-intensity, narrow electron-beams projected by remotely positioned electron guns.
Electron-beam furnances are known in the art and are used for melting materials, primarily metals, to cast ingots therefrom. By using such furnaces, ingots of exceptionally high purity may be obtained, owing primarily to effective outgassing and vaporizing of impurities in the material during the melting process.
In general, an electron-beam furnace includes a vacuum chamber enclosing a crucible or mold, at least one electron gun providing an energetic beam of electrons, means for directing the beam at the material to be melted (melt stock) and means for feeding the melt stock into the beam. The material thus melted is received by the crucible where it cools and solidifies into an ingot. During typical operation of such furnaces, the ingot is progressively drawn from the bottom of the crucible while the melt stock is being continuously fed into the beam.
Owing to the presence of impurities in the melt stock, bombardment of the melt stock by the electron-beam normally causes the release of substantial quantities of gaseous and vaporized impurities, frequently in violent bursts at random intervals. In addition to such evolved gaseous matter, there is also the splattering of molten material caused by the release of the gaseous matter in violent bursts. Therefore, it is generally desirable to position the electron gun of the furnace at a relatively large distance away from the region of electron bombardment and out of the way of the evolving gaseous matter and the splattering molten material. In this manner, damage to the electron gun caused by the condensation of the gaseous matter and the collection of molten material on the gun is avoided.
In prior art electron-beam furnaces, such as those described in U.S. Pat. Nos. 3,189,953; 3,105,275; 3,080,626; and 3,087,211, a remotely positioned, relatively large annular electron gun is used for providing a conelike electron-beam. The beam is focussed and guided into the opening of the crucible by a relatively strong magnetic field produced by one or more coils. The coils are positioned coaxially with respect to the crucible, with one coil wound around the crucible itself. Since the electron-beam tends to converge in regions of high magnetic flux density, the magnetic field used in prior art furnaces concentrates and directs a broad beam generally into the crucible in the direction of its axis but permits no scanning motion of the beam.
For casting ingots of very high purity, it is highly advantageous to provide the furnace with a large-area, trough-shaped hearth in which to melt the material. The use of such a hearth results in the exposure of a larger surface area of the molten material to the vacuum ambient, and therefore provides more effective refining of the impurities from the material through outgassing or vaporization. The hearth is typically provided with a pouring spout, from which molten material may flow into an appropriately situated crucible.
The magnetic beam guidance systems of prior art electron-beam furnaces are not suitable for use in a furnace having a hearth, because such systems are generally not capable of providing an electron-beam having a sufficiently large cross-sectional area to cover the entire hearth and crucible. However, the material in the hearth and crucible may be heated by scanning the hearth and crucible with one or more high-intensity, relatively narrow electron-beams provided by separate, remotely positioned electron guns, each having its own deflection system for sweeping the beam across the hearth and crucible in a desired manner.
When melting most metals in the hearth, the scanning of the electron-beams across the hearth and crucible may be accurately controlled by deflection systems at the remotely positioned electron guns without difficulty. It has been found, however, that during the melting of certain metals, such as alumino-thermically reduced columbium (Niobium) or zircaloy-2, the beams cannot be accurately scanned over the hearth and crucible, owing to the tendency of the beams to deviate erratically from their intended targets. Such erratic beam behavior may be caused by charge fluctuations in a plasma formed in the vicinity of the hearth and the crucible when the gaseous matter evolving from the electron bombarded metal is ionized by the electron-beams. Such charge fluctuations can cause random local deflections of the electron-beams as they pass through the plasma. Since the problem of erratic beam behavior makes the heating of the material in the hearth and crucible difficult to control, a need exists for an electron-beam furnace configuration which provides for stabilization of electron-beams projected from remotely positioned electron guns so as to permit accurate control of the scanning of each beam by the deflection system of its respective gun.